Ligand-gated ion channels
The ligand-gated ion channel is composed of 5 subunits: 2 α-subunits, 2 β-subunits and a γ-subunit. In rest, an ion channel is closed, when an agonist binds the channel, it opens within milliseconds, resulting in a direct ion exchange. The conformational changes due to binding of the ligand open the channel. Sometimes two agonists must bind before a channel opens. An antagonist usually keeps the channel closed, but some agonists can also act as a channel blocker. Examples of ion channels are nicotinic acetylcholine receptors and GABA-A receptors.
Ion channels are important drug targets in the treatment of cardiac diseases.
Sodium channel blockers are an important drug group. Cardiac and skeletal muscle and neuronal sodium channels differ in structure and require very different concentration of drugs to block. Local anaesthetics and some anti-arrhythmic drugs block sodium channel during depolarisation. The overall effect is a delay in conductance and a decrease in excitability.
Calcium channels open during depolarisation. Drugs that inhibit this opening are called calcium antagonists (diltiazem, verapamil) and thus decrease conductance and contraction of the heart.
Potassium channels open to induce hyperpolarisation.